Selected Correspondence:
Letter 39 Spinoza to [the Elwes translation shows this as Christian Huyghens, Shirley and others show it as John Hudde.]

Distinguished Sir, -The demonstration of the unity of
God, on
the ground that His nature involves necessary
existence, which you
asked for, and I took note of, I have been prevented by various
business from sending to you before. In order to accomplish my
purpose, I will premise-

II. That no definition
can involve or express a multitude or a given
number of individuals, inasmuch as it involves and expresses nothing
except the nature of the thing as it is in itself. For instance, the
definition of a triangle includes nothing beyond the simple nature of a
triangle; it does not include any given number of triangles. In like
manner, the definition of the
mind as a thinking thing,
or the definition of
God as a perfect Being, includes
nothing beyond the natures of the mind and of
God,
not a given number of minds or gods.

From these premisses it follows, that if any given
number of individuals exists in nature, there must be
one or more causes, which have been able to produce
exactly that number of individuals, neither more nor
less. If, for instance, there existed in nature twenty men
(in order to avoid all confusion, I will assume that these
all exist together as primary entities), it is not enough to
investigate the cause of human nature in general, in
order to account for the existence of these twenty; we
must also inquire into the reason, why there exist exactly
twenty men, neither more nor less. For (by our third
hypothesis) for each man a reason and a cause must be
forthcoming, why he should exist. But this cause (by our
second and third hypotheses) cannot be contained in
the nature of man himself; for the true
definition of man
does not involve the number of twenty men. Hence (by
our fourth hypothesis) the cause for the existence of
these twenty men, and consequently for the existence of
each of them, must exist externally to them. We may
thus absolutely conclude, that all things, which are
conceived to exist in the plural number, must
necessarily be produced by external causes and not by
the force of their own nature. But since (by our second
hypothesis) necessary
existence appertains to the nature
of God, His
true definition
must necessarily include
necessary existence:
therefore from His true definition
His necessary existence
must be inferred. But from His
true definition (as I have already demonstrated from our
second and third hypotheses) the necessary existence of
many gods cannot be inferred. Therefore there only
follows the existence of a single
God. Which was to be
proved.

This, distinguished Sir, has now seemed to me the
best method for demonstrating the proposition. I have also proved it
differently by means of the distinction between
essence and
existence;
but bearing in mind the object you mentioned to me,
I have preferred to send you the demonstration given above. I
hope it will satisfy you, and I will await your reply, meanwhile
remaining, &c.